1. Field of the Invention
The invention relates to a process for producing solids of silanol salts from alkoxysilanes, alkali metal hydroxide and water, in which water and alcohol are removed in a fixed bed.
2. Description of the Related Art
Alkali metal organosiliconates such as potassium methyl siliconate have already been in use for decades for hydrophobization, in particular of mineral construction materials. Owing to their good solubility in water they can be applied in the form of an aqueous solution to solids, where, after evaporation of the water, they form firmly adhering, permanently water-repellent surfaces under the influence of carbon dioxide. Since they comprise virtually no hydrolytically cleavable organic radicals, curing advantageously takes place without the release of undesirable volatile, organic secondary products.
The preparation of alkali metal organosiliconates, in particular potassium and sodium methyl siliconates, has been described many times. In most cases, the focus is on the preparation of ready-for-use and storage-stable, aqueous solutions. For example, DE 4336600 claims a continuous process starting from organotrichlorosilanes via the intermediate organotrialkoxysilane. Advantages of that process are that the secondary products hydrogen chloride and alcohol that form are recovered, and the siliconate solution that forms is virtually free of chlorine.
Ready-for-use construction material mixtures such as cement or gypsum renders and fillers or tile adhesives are mainly supplied to the construction site in the form of powders in bags or silos and are only mixed with the mixing water on site.
There is required for that purpose a solid hydrophobizing agent which can be added to the ready-for-use dry mixture and develops its hydrophobizing action in a short time only upon addition of water during application on site, for example on the construction site. This is called dry-mix application. Organosiliconates in solid form have proved to be very efficient hydrophobizing additives for that purpose. Their use is described, for example, in the following specifications:
Application PCT/EP2011/061766 claims solid organosiliconates having a reduced alkali metal content. Their preparation is carried out by hydrolysis of alkoxy- or halosilanes with aqueous alkali metal hydroxide solution and azeotropic drying of the resulting, optionally alcoholic-aqueous siliconate solution with the aid of an inert solvent as an entrainer.
U.S. Pat. No. 2,567,110 describes access to neutral (poly)siloxanes starting from alkali metal sil(ox)anolates and chlorosilanes. Example 1 describes the preparation of sodium methyl siliconate by reaction of a monomethylsiloxane hydrolyzate with a molar equivalent of sodium hydroxide solution in the presence of ethanol. The solid is isolated by distilling off the solvent and is then dried at 170° C. to a constant weight. Such a process for isolating solids is unworkable on an industrial scale because there, deposits that adhere firmly during the concentration by evaporation form on the walls of the reaction vessel.
A further disadvantage of the hitherto described processes of concentration by evaporation in the isolation of the solid is the fact that alkali metal siliconates decompose thermally, which constitutes a reaction safety problem. For example, potassium methyl siliconate (K:Si=1:1) decomposes above 120° C. in a highly exothermic reaction of 643 J/g with the loss of the methyl group. Under adiabatic conditions, the temperature rises to over 300° C. Consequently, it is also to be assumed that thermal decomposition occurs in the process claimed in DE 1176137 for drying an aqueous siliconate solution at 350-400° C. on a rotating hotplate. Irrespective thereof, such high temperatures require specific, expensive materials and complex safety measures in particular when flammable solvents are present. Moreover, starting from predominantly or purely aqueous solutions of the alkali metal siliconates, a very large amount of energy is required for the evaporation of the solvent water, which impairs the economy of the process or is too complex in terms of apparatus for conversion to an industrial scale.
U.S. Pat. No. 2,438,055 describes the preparation of siliconates as hydrates in solid form. In that document, the hydrolyzate of a monoorganotrialkoxysilane or of a monoorganotrichlorosilane is reacted with 1-3 molar equivalents of alkali metal hydroxide in the presence of alcohol. The siliconates formed as hydrates are crystallized out by evaporating off the alcohol or by adding corresponding non-polar solvents. In Example 1, the preparation of solid sodium methyl siliconate hydrates is described: to that end, 1 molar equivalent of methyltriethoxy-silane is reacted with 1 molar equivalent of sodium hydroxide in the form of saturated sodium hydroxide solution (i.e. 50 wt. %). Methanol is added to the solution in order to crystallize the siliconate. Evidently only a portion of the siliconate thereby precipitates. In fact, a further solid is isolated by concentration of the mother liquor by evaporation, which solid exhibits a 21% weight loss upon drying over P2O5 at 140° C. Nothing is said about the relative proportions.
In U.S. Pat. No. 2,803,561 alkyltrichlorosilane is hydrolyzed to the corresponding alkylsilicic acid, which is subsequently reacted with alkali metal hydroxide to give an aqueous solution of alkali metal siliconate, which is stabilized by addition of up to 10% alcohol or ketone. How the drying of the siliconate is carried out is not described. The use of the dried siliconate for the hydrophobization of gypsum is mentioned.